Excel 2010 VBA - difficulty creating a custom function

Function addup_numbers(firstnum, lastnum)

    addup_numbers = firstnum + lastnum  

End Function

Note the + rather than the :

or just

Function addup_numbers(firstnum, lastnum)

    addup_numbers = firstnum + lastnum

End Function

Basically in user defined functions you usually (always?) need to use VBA code rather than sheet formulas.

OK, UDF, my favorite topic, love 'em.

Start by declaring variables in the function

E.G:

Function addup_numbers(firstnum as double, lastnum as double) as double

    addup_numbers = firstnum + lastnum

End Function 

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I am guessing that you are trying to sum all cells between a range not just the first and last cell? or?

As you have used the colon ':' I am guessing you are trying to do a range:

Function addup_numbers(allnums as range) as double
 
for each c in allnums
if isnumeric(c) then
  addup_numbers = addup_numbers + c.value
end if
next
End Function

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NOTE:
if you want to add up numbers between two cells such as StartCell and EndCell and then use VBA to call the remaining cells... I wouldn't...
Try to avoid a function which does not include the range or cells involved in the formula in brackets, as if you change the cells that are referenced but not in the brackets then the result of the formula will not calculate automatically.

Barman:
Thank you.
What I'm trying to understand is the VBA syntax used in implementing the sum function within my user-created function.  
I'm trying to add the following numbers:
The range a1 up to and including a10.  
Just a simple sum function to add up a contiguous range.  
My purpose in posting the question was I wanted to know how come it rejected my use of the colon!  I know that there may be faster and better ways of getting this job done but I want to make my solution work.  I want to understand the syntax of my use of the SUM function within a UDF.  This is purely a learning exercise.
I do not merely want to add firstnum  and lastnum together.  I should have more accurately stated my arguments as first_num_in_range and last_num_in_range

As I have stated in earlier posts, it is not good practice to do this as the function result will not calculate automatically if any of the values between first and last number in the range change.
However to do what you wish:

Function addup_numbers(first_num_in_range As Range, last_num_in_range As Range) As Double
Dim allnums As Range

Set allnums = Range(first_num_in_range, last_num_in_range)
Debug.Print allnums.Address

For Each c In allnums
If IsNumeric(c) Then
  addup_numbers = addup_numbers + c.Value
End If
Next

End Function

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Thanks Barman.
Very useful solution.  It works perfectly.
Question:  How can I make the sum function calculate a range in a UDF
Where do I put quotes or ampersands, etc.  
That's all I want.
I know this is a sucky solution to a simple problem but I'm just trying to understand syntax of VBA i.e. how and where to place quote marks. ampersands, etc.

Function addup_numbers(first_in_range,  last_in_range)

    addup_numbers = sum(first_in_range : last_in_range)    

End Function

You don't.

Sum is a worksheet function.

Function addup_numbers(first_num_in_range As Range, last_num_in_range As Range) As Double

addup_numbers = WorksheetFunction.Sum(Range(first_num_in_range, last_num_in_range))

End Function

Nice.